1. Field of the Invention
This invention relates to a method of manufacturing a flexible optical fiber bundle, and more particularly to a method of manufacturing an optical fiber bundle comprising a plurality of optical fibers which are connected together at the ends thereof and are disconnected from each other at the intermediate portion between the connected ends to give flexibility to the optical fiber bundle at the intermediate portion thereof.
2. Description of the Prior Art
When an optical fiber bundle is used for guiding an image, the arrangement of the end faces of the individual optical fibers at one end of the bundle must correspond to that at the other end thereof. When the optical fiber bundle for the image guide is used in an endoscope, for example, the bundle should have a flexible intermediate portion with the optical fibers constituting the bundle connected together at the ends thereof. In such an optical fiber bundle, the intermediate portions of the individual optical fibers are disconnected from each other to give flexibility to the intermediate portion of the bundle. There have been proposed various methods of manufacturing an optical fiber bundle in which the individual optical fibers are connected together at the ends thereof and disconnected from each other at the portion between the ends.
In one method, core glass material having a high refractive index is introduced into the inner crucible of a double crucible and cladding glass material having a low refractive index is introduced into the outer crucible of the same. The double crucible is heated up to an appropriate temperature and the melt glass is drawn through orifices in the bottom of the crucible from the inner and outer crucibles to obtain an optical fiber consisting of a core and a cladding therearound. The optical fiber thus obtained is rolled up to form a plurality of loops contiguously arranged in a single array in the axial direction thereof. The loops are bonded together by means of adhesive at a part thereof. Subsequently, a second array of the same kind is stacked on the first array and loops of the second array are bonded together at the same part of the loops and to the bonded part of the first array. These steps are repeated to obtain a looped optical fiber bundle having a desired thickness. Then the looped optical fiber bundle is severed at the center of the bonded part along a line perpendicular to the longitudinal direction of the optical fibers, and the cut surfaces are polished.
In this method, a desired final thickness, e.g., 20.mu. of the optical fibers is obtained in one heating operation, and therefore, very thin optical fibers must be handled in the subsequent step, i.e., the step of arranging the optical fibers. This step requires skill and there is a high possibility of severing the optical fiber during the step, which results in a low yield and an increased manufacturing cost.
Another method involves use of an acid step. In this method, core glass material having a high refractive index is introduced into the innermost crucible of a triple crucible, cladding glass material having a low refractive index and a high resistance to acids is introduced into the middle crucible of the same, and acid-soluble glass material having an expansion coefficient and a viscosity similar to those of the core glass and the cladding glass is introduced into the outermost crucible of the same. The triple crucible is heated up to an appropriate temperature and the melt glass is drawn through orifices in the bottom of the triple crucible from three crucibles thereof, thereby obtaining a three-layer optical fiber comprising an innermost layer of the core glass an intermediate layer of the cladding glass and an outermost layer of the acid-soluble glass and having a diameter of about 200.mu.. The three-layer optical fiber is cut into pieces of an appropriate length, e.g., 400 mm. A number (e.g., 10,000) of the pieces are orderly arranged in a jacketing tube of acid-soluble glass, and are fused together. The assembly is then further heated and stretched to reduce the diameter of each optical fiber piece to about 1/15. The solid optical fiber bundle thus obtained is provided with acid resistant coatings at the end portions thereof, and then brought into contact with an acid (e.g., nitric acid) to elute the acid-soluble glass from the intermediate portion thereof. In this method, the step of arranging the optical fibers can be more easily carried out with a less possibility of severing the fibers than with the previously described method, since the thickness of the fibers handled in the step is larger. Further, since the fibers are fused together subsequently, there is little possibility of their being severed after the step of arranging them. Accordingly, with this method, the yield of the products can be increased and the manufacturing cost can be reduced compared with the previously described method.
However, this method is disadvantageous in that a jacketing tube of acid-soluble glass, which is poor in workability compared with soda-lime glass, must be used. Because of the poor workability of the acid-soluble glass, fluctuation in the internal diameter of the jacketing tube in its longitudinal direction is very large. When such a jacketing tube is used, the orientation of the individual optical fibers is adversely affected so that the image transmitting characteristics of the obtained optical fiber bundle is significantly lowered.
In order to overcome the above problem, there has been proposed another method intended to improve on the method described immediately above (Unexamined Japanese Patent Publication No. 52(1977)-42737 dated Apr. 2, 1977). In the improved method, the jacketing tube of the acid-soluble glass containing there in a number of the three-layer optical fiber pieces is inserted into an over-jacketing glass tube which is larger than the jacketing tube both in diameter and length. The over-jacketing glass tube is connected to an evacuating means for internal evacuation thereof. Then the over-jacketing glass tube is heated to fuse the three-layer optical fiber pieces together and subsequently removed therefrom. In this method, the over-jacketing glass tube may be of ordinary soda-lime glass and thus can be formed to a sufficient dimensional accuracy so that the optical fiber pieces can be properly oriented. However, the over-jacketing glass tube must be mechanically removed, which significantly adds to the number of the manufacturing steps.